Polarization insensitive microbend fiber gratings and devices using the same

ABSTRACT

A microbend-induced fiber grating is formed from a section of optical fiber configured to exhibit “splitting” between the resonant wavelengths supported by the TE and TM components of the LP 1m  mode and the resonant wavelength supported by the odd/even HE 2m  components of the LP 1m  mode. Since only the TE and TM components are polarization dependent, by splitting and shifting the resonant wavelengths for these modes away from a system-desired wavelength(s) supported by the odd/even HE modes, a polarization insensitive microbend-induced fiber grating can be formed. A fiber core configuration including a central core region, trench and ring is formed to exhibit a large radial gradient in core refractive index profile, with a significantly steep transition between the ring index and the trench index, to provide the desired splitting between the (undesired, polarization sensitive) TE/TM modes and the HE mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.10/914,208, filed Aug. 9, 2004 and allowed on Oct. 2, 2006 now U.S. Pat.No. 7,177,510.

TECHNICAL FIELD

The present invention relates to microbend fiber gratings and, moreparticularly, to forming polarization insensitive microbend fibergratings by intentionally separating the HE and TE/TM vector modes in auniquely configured fiber such that only the HE modes are present forthe resonant wavelength(s) of interest.

BACKGROUND OF THE INVENTION

Tunable broadband mode converters play an important role in wavelengthdivision multiplexed (WDM) optical communication systems. They may beused, for example to dynamically convert a lightwave signal propagatingin one mode of a “few mode” fiber into another spatial mode. Suchcoupling is attractive to alter the path the lightwave signal takes,since the alternate path (defined by the other spatial mode in thefiber) may have preferred dispersion, nonlinearity or amplificationproperties. An example of this is a higher-order-mode (HOM) dispersioncompensator, where light in an entire communication band is switchedfrom an incoming LP₀₁ mode to a higher-order mode such as the LP₁₁ orLP₀₂ mode.

In a general sense, long-period gratings (LPGs) are mode-conversiondevices that provide phase-matched coupling to transfer power from onemode of an optical fiber to another. This has proven to be especiallyuseful for coupling between a guided mode and a cladding mode ofordinary transmission fibers so as to create a wavelength-selectiveloss. In optical communication systems, LPGs have been used extensivelyfor realizing devices that offer wavelength-selective attenuation of aWDM communication signal. Dynamic tuning of the spectral characteristicsof LPGs has been proposed and a variety of dynamic tuning techniqueshave been demonstrated. In particular, LPGs that couple the core mode toa cladding mode can be dynamically tuned by modulating the refractiveindex of an outer or inner cladding material that interacts with thecladding mode of the fiber. A microbend-induced fiber grating (MIG) isone type of dynamic LPG where the perturbation in refractive index isinduced by periodic “microbending” of the fiber. In particular, thecoupling strength of a MIG is tunable by changing the strength of apropagating acoustic wave or by changing the pressure applied to thefiber (e.g., pressing the fiber with a corrugated surface of a definedperiodicity).

As will be described in detail below, MIGs have several advantageousdevice applications. For example, when one of the co-propagating modesis the fundamental mode of a single mode fiber and the other is acladding-guided mode, MIGs yield wavelength-dependent loss spectra whenbroadband light is transmitted through the single mode fiber.Wavelength-dependent loss is known to be useful for several deviceeffects, such as gain equalization filters, spectral shapers forbroadband lightwave devices, amplified spontaneous emission filters,loss filters for stabilizing the operating wavelength of fiber lasers,etc. On the other hand, if both of the co-propagating modes are guidedin the core region of a fiber, MIGs can be used to realize efficientmode conversion, as noted above, which has applications such ashigher-order mode conversion, variable optical attenuation, etc.

One drawback to the use of MIGs is their inherent polarizationsensitivity, even when the grating is induced in a perfectly circularfiber. The mode depictions in FIGS. 1 and 2 can be used to explain thisphenomenon. A microbend-induced fiber grating, as noted above, willcouple a circularly symmetric and polarization degenerate mode (such asthe LP₀₁ mode shown in FIGS. 1( a) and (b)) with anti-symmetric LP_(1m)modes (such as the LP₁₁ mode of FIGS. 1( a) and (b)), where m definesthe radial order of the anti-symmetric mode. Referring to FIG. 2, theLP₁₁ mode is shown as possessing a four-fold degeneracy including thevector modes TE₀₁, TM₀₁ and the odd and even HE₂₁ modes. In any fiberwaveguide possessing radial index variations (which are necessary todefine a core/cladding boundary), these four modes are known to exhibitslightly different propagation constants. Thus, coupling with amicrobend fiber grating of a given grating period Kato results inexhibiting slightly different resonant wavelengths for each one of thedifferent modes. Since different polarization orientations of thefundamental mode will result in different excitation levels for the fourmodes, the resulting coupling spectrum will also be polarizationdependent—an unwanted result since it severely restricts theapplicability of MIGs in fiber optic systems, where a “polarizationinsensitive” response is often necessary condition.

Prior attempts at reducing the polarization sensitivity of MIGs havegenerally fallen into three classes: (1) inducing microbends along twoorthogonal transverse axes of the fiber, in one case by helicallywinding thin wires around a fiber to generate circularly symmetricmicrobends; (2) forming MIGs in extremely thin fibers (results incoupling only to the odd/even HE modes) and (3) introducing polarizationdiversity into the system, using external components to compensate forpolarization-dependent losses. Looking at the first solution, it hasbeen found to be limiting in the sense that it involves the precisionmachining of expensive and complex corrugated blocks with tight angulartolerances. Additionally, it is necessary to ensure that no polarizationrotation occurs as the light traverses from one set of microbends to anorthogonal set of microbends. The use of helical microbends, as alsoproposed, requires individual assembly for each device (with each devicerequiring a high level of precision) and cannot provide “strengthtuning”—the fundamentally attractive feature of MIGs. The use ofextremely thin fibers, as required in the second class of solutions, isnot practical for “real world” system applications and can only be usedwith acousto-optic configurations since the action of pressing acorrugated block against an extremely thin fiber introduces a host ofreliability and yield issues. The third class of solutions (polarizationdiversity) requires the use of a device such as a Faraday rotator mirrorto rotate the state of polarization (SOP), in association with acirculator and polarization beam splitter to form a pair of orthogonalsignals. Indeed, a pair of essentially identical MIGs would be required,each acting on a separate one of the orthogonal components. This schemeis considered to add substantial loss, as well as cost and size, to thesystem.

Thus, a need remains in the prior art for a microbend-induced fibergrating that is polarization insensitive and useful in a variety ofsystem applications, providing polarization insensitivity regardless ofthe configuration used to induce the microbends in the fiber (e.g.,acousto-optical fiber, corrugated blocks, permanently etched gratings,etc.).

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the presentinvention, which relates to microbend fiber gratings and, moreparticularly, to forming polarization insensitive microbend fibergratings by intentionally separating the HE and TE/TM vector modes in auniquely configured fiber such that only the HE modes are present forthe resonant wavelength(s) of interest.

The present invention is based on the discovery that a microbend fibergrating can be made inherently polarization insensitive if lightwavecoupling is limited to occur only into the HE odd/even modes. Typically,the propagation constants of the TE₀₁, TM₀₁ modes are slightly differentfrom the propagation constant of the pair of HE₂₁ modes (where theodd/even HE modes exhibit an identical propagating constant). Thedifference in propagation constants is very small, and the correspondingwavelengths of resonance for a uniform microbend induced fiber gratingformed in such fibers differ by approximately 0.2 to 5.0 nm for variousstates of polarization (SOPs) of the input light. While this difference(hereinafter referred to as “wavelength splitting”) is relatively small,it is large enough to induce polarization dependent losses of 10 dB ormore.

In accordance with the present invention, a fiber is configured toprovide the desired wavelength splitting by designing the refractiveindex profile of the fiber core to exhibit a substantially large radialgradient. In particular, the fiber core is defined as including acentral core region, a trench surrounding the central core region and aring surrounding the trench. Each portion of the core is defined by itsrefractive index (Δn) and radius r, the refractive index being definedas a “refractive index difference” with respect to the refractive indexof silica (defined as “0”), and the radius thus defining the thicknessof the particular region. The following parameters are used as designrules that are simultaneously satisfied to form a fiber that exhibitsthe desired wavelength splitting of the present invention: (1) therefractive index difference of the ring (Δn_(r)) is selected to begreater than 0.015, with a sufficiently “steep” transition (no more than1 μm) between the refractive index of the trench and the ring; and (2)the refractive index difference of the central core region (Δn_(c)) ismaintained to be approximately three-quarters the value of therefractive index difference of the ring Δn_(r) (i.e.,Δn_(c)≈0.75*Δn_(r)).

In one embodiment of the present invention, the fiber may be configuredto also exhibit a “turn-around point” (TAP) condition, as discussed inmy co-pending application Ser. No. 10/234,289, filed Mar. 4, 2004, wherethe TAP condition has been found to provide for a relatively largebandwidth of operation, and can therefore yield grating resonances withvery large bandwidths.

It is a significant aspect of the present invention that such awavelength splitting microbend fiber grating may be formed using eitherthe acousto-optic or corrugated element tuning technique (as well asused for fixed wavelength, non-tunable microbend fiber gratings).

Other and further aspects and embodiments of the present invention willbecome apparent during the course of the following discussion and byreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIGS. 1( a) and (b) illustrate radial plots (FIG. 1( a)) and intensityplots (FIG. 1( b)) for the LP₀₁ and LP₁₁ modes;

FIG. 2 contains, in FIG. 2( a) the vector presentation of the four modesof the LP₁₁ mode, along with the pair of scalar representations in FIG.2( b);

FIG. 3 contains a graph of the spectra (for both parallel andperpendicular polarizations) for a set of three phase matching curves;

FIG. 4 illustrates, in idealized form, the refractive index profile fora polarization insensitive microbend-induced fiber grating MIG) of thepresent invention;

FIGS. 5( a)-(c) illustrate experimentally measured properties of apolarization insensitive MIG of the present invention;

FIG. 6 is a plot of polarization intensities for an exemplary inputsignal, “forces” to follow the perpendicular and parallel states ofpolarization;

FIGS. 7( a) and (b) contain the relative index plots and phase matchingcurves for a second embodiment of a polarization insensitive MIG formedin accordance with the present invention;

FIGS. 8( a)-(d) illustrate a set of four exemplary techniques forforming/inducing microbends into a fiber formed in accordance with thepresent invention;

FIG. 9 illustrates a system comprising a cascaded set of polarizationinsensitive MIGs formed in accordance with the present invention;

FIGS. 10( a)-(c) illustrate various embodiments for using thepolarization insensitive MIG of the present invention as a variableoptical attenuator;

FIGS. 11( a) and (b) illustrate the utilization of the polarizationinsensitive MIG as an optical switch; and

FIGS. 12( a) and (b) illustrate the utilization of a set of polarizationinsensitive MIGs of the present invention as mode converters (eitherstatic or dynamic mode converters).

DETAILED DESCRIPTION

As will be discussed in detail below, the optical fiber within which amicrobend grating is formed is intentionally designed, in accordancewith the present invention, such that the propagation constants of theTE_(0m) and TM_(0m) modes are substantially separated from thepropagation constant of the odd/even HE_(2m) mode. As a result, theresonant wavelengths associated with the TE_(0m) and TM_(0m) modes willbe significantly different/separated from the resonant wavelength of theHE_(2m) mode, where the resonant wavelength of the HE_(2m) mode is thendefined as the transmission wavelength for the optical system. For theremainder of the discussion, it will be presumed that radial mode m willbe the first order mode, where m=1. It is to be understood, however,that the principles of the present invention are equally applicable tohigher order modes.

It has been found that fibers with substantially large radial gradientsin their refractive index profiles (i.e., where a significant portion ofthe power of the LP₁₁ mode resides close to regions of the fiber wherethe refractive index varies rapidly with radial position) lead to thedesired large wavelength splitting required for polarizationinsensitivity. Thus, the present invention is embodied in a number ofdifferent fiber configurations that provide this rapid change in radialrefractive index.

FIG. 1, as referred to briefly above, illustrates in FIG. 1( a) theradial plots of intensity for the LP₀₁ and LP₁₁ modes, respectively, inan optical fiber with a core designed to support these two modes. FIG.1( b) contains the corresponding two-dimensional intensity plots forthese same LP₀₁ and LP₁₁ modes. As is evident from FIG. 1, the LP₀₁ modehas a circularly symmetric field profile, while the LP₁₁ mode contains anull in its center, and is circularly antisymmetric. This representationof “linearly polarized” (LP) modes in an optical fiber is onlyapproximately correct, since it treats the electric field as a scalarinstead of a vector quantity. For example, the propagation constants β₀₁of both the parallel and perpendicular polarizations of the LP₀₁ modeare identical—thus, the propagation characteristics of the fundamentalmode are inherently insensitive to the state of polarization of thesignal passing therethrough. However, as mentioned above, it is wellknown that the LP₁₁ modes are actually linear combinations of fourdistinct vector modes, namely the TE₀₁, TM₀₁ and the even and odd HE₂₁modes. The vector representations for these four modes are illustratedin FIG. 2( a), along with their linear combinations in FIG. 2( b),resulting in the two types of scalar LP₁₁ modes. The arrows of FIG. 2(a) represent the orientation of the electric field. When only the scalarwave equation is considered, these modes have identical propagationconstants. However, when the full vectorial wave equation is considered,the four modes have distinct propagation constants. That is,

β_(TE₀₁) ≠ β_(tm₀₁) ≠ β_(HE₂₁), and  β_(HE₂₁)^(even) = β_(HE₂₁)^(odd),which shows that while the TE, TM and HE modes have differentpropagation constants, the even and odd (and thus any rotationallyvariant version of the) HE modes are degenerate.

The difference in propagation constants for these various modes isrelatively small, and can be obtained by first solving the scalar waveequation, which yields the propagation constant for the LP₁₁ mode, andthen applying the first-order perturbation theory to obtain thefirst-order vector corrections to this field, as shown below:β_(vector)=β_(scalar)+δβ_(pert).

This difference has serious implications for the performance ofmicrobend induced fiber gratings (MIGs), leading to the discovery ofpolarization insensitivity as a result of mode splitting (“wavelengthsplitting” as a function of mode) in accordance with the presentinvention. In particular, MIGs couple the fundamental mode withcircularly antisymmetric LP₁₁ modes, and the resonant wavelength ofoperation is closely tied to the propagation constants of differentmodes by the following resonant condition:

${\lambda_{res} = {\Lambda \cdot \left( {n_{01} - n_{1m}} \right)}},{{{{and}\mspace{14mu}\beta_{01}} = {\frac{2\pi}{\lambda_{res}}n_{01}}};{\beta_{1m} = {\frac{2\pi}{\lambda_{res}}n_{1m}}}},$where n₀₁ and n_(1m) are the effective indices of the LP₀₁ and LP_(1m)modes, respectively, and are related to the propagation constants (asshown above), and Λ is the grating period.

Since each vector component in the LP_(1m) mode group has a slightlydifferent propagation constant, it follows that the resonant wavelengthsare also different for each of the modes. Further, the TE_(0m) andTM_(0m) modes are, by definition, polarization sensitive, and areexcited by the grating only for certain input states of polarization ofthe LP₀₁ mode. On the other hand, the degenerate HE_(2m) pair haselectric field vectors pointing in all directions within thecross-sectional plane of a fiber, and are thus excited by any inputstate of polarization for the LP₀₁ mode. This result implies that theresonance generated by a microbend grating coupling the fundamental modeto an LP_(1m) mode may exhibit three different resonances (i.e., aresonant condition at three different wavelengths), two of which wouldbe strongly polarization dependent.

In practice, since these three modes have almost identical propagationconstants, their resonances merge slightly, yielding two distinct (buthighly polarization sensitive) resonances. FIG. 3 illustrates thespectra of microbend grating resonances excited in an exemplary fiber,with a 600 μm grating period designed to couple the fundamental modewith the cladding guided LP₁₃ mode, for a variety of strengths andpolarization states. The phase matching curves for the TE₀₃, HE₂₃ andTM₀₃ modes are distinct, but similar, for the reasons discussed above,with their resultant spectra shown as curves 310, 320 and 330. Thespectra illustrate resonances of increasing strength when the electricfield vector of the fundamental mode (LP₀₁) is parallel to the plane inwhich the microbends are induced. Likewise, curves 340, 350 and 360 showresonances of increasing strength when the electric field vector of thefundamental mode is perpendicular to the plane in which the microbendsare induced. This difference in resonances between the parallel andperpendicular polarizations clearly shows both the significant advantage(independent, strength-only tuning capability) and the fundamentallimitation (inherent polarization sensitivity) of microbend fibergratings. Note that the polarization-dependent losses (PDL) can be ashigh as 10 dB for a 20-dB resonance. This polarization dependence ofMIGs may thus be characterized in terms of the spectral separationbetween resonances. For the given example of MIGs in conventional priorart fibers, the resonances are separated by approximately 1 nm, wherethis value may vary between as low as 0.3 nm and several nm, dependingon the type of fiber used. Regardless of fiber type, however, all ofthese values of resonance separation lead to unacceptable levels of PDL,and hence this is an inherent problem of all MIGs.

In accordance with the present invention, therefore, a novel class offiber designs is proposed that enables polarization-independentmicrobend fiber gratings and is based on the discovery that fibers canbe designed to accurately control the difference in propagationconstants δβ_(pert), as defined in the above equations, between thedifferent HE_(2m), TE_(0m) and TM_(0m) modes. For any fiber waveguide,the core is defined by:n ²(r)=n _(co) ²[1−2Δ·ƒ(r)],where n_(co) is defined as the peak index of refraction in the core, ris the radial coordinate defining the distance from the center of thecore, Δ is the “relative index contrast”, as defined by the relation(n_(co)−n_(cl))/n_(co), n_(cl) being the refractive index of thecladding, and ƒ(r) is defined as the normalized profile of therefractive index. Additionally, the perturbations to the scalarpropagation constant are given by:

$\begin{matrix}{{\delta\beta}_{{TE}_{01}} = 0} \\{{\delta\beta}_{{TM}_{01}} = {2\left( {I_{1} + I_{2}} \right)}} \\{{{\delta\beta}_{{HE}_{21}}^{even} = {{\delta\beta}_{{HE}_{21}}^{odd} = \left( {I_{1} + I_{2}} \right)}},}\end{matrix}$where the quantities I₁ and I₂ are related to the refractive indexprofile of a fiber by the following relations:

$I_{1} = {\int{{r \cdot {E_{l\; m}(r)} \cdot \frac{\partial{E_{l\; m}(r)}}{\partial r} \cdot \frac{\partial{f(r)}}{\partial r}}{\mathbb{d}r}}}$${I_{2} = {\int{{{E_{l\; m}^{2}(r)} \cdot \frac{\partial{f(r)}}{\partial r}}{\mathbb{d}r}}}},$where E_(lm)(r) is defined as the field profile for the mode withindices “l” and “m”.

Since the provision of optical coupling with microbend-induced fibergratings is restricted to antisymmetric modes that comprise polarizationdependent components (i.e., the TE_(0m) and TM_(0m) modes) as well asthe polarization insensitive HE_(2m) mode, a polarization-insensitivemicrobend-induced fiber grating of the present invention is thus formedby using a fiber where coupling occurs only to the polarizationinsensitive component of this triplet, namely, the HE_(2m) mode (atleast within the spectral range of interest). Since the spectral rangeis typically 20 nm or larger, a fiber that is designed to haveresonances of the three modes separated by 20 nm or more will lead topolarization-insensitive MIG.

FIG. 4 illustrates, in an ideal form, the refractive index profileassociate with realizing a “wavelength splitting” fiber that may be usedin forming a polarization-insensitive MIG in accordance with the presentinvention. The idealized fiber index profile, as shown, is defined by acentral core region 10 of refractive index Δn_(c) and radius r_(c), atrench 12 surrounding core region 10 with a refractive index of Δn_(t)and radius r_(t), and a ring 14 surrounding trench 12, with a refractiveindex of Δn_(r) and radius r_(r). The refractive indices are eachdescribed in terms of “refractive index difference” from the silicacladding reference value of zero. The design rules for the fiber aregoverned by the relations discussed above, where it follows that theperturbations, δβ_(TM) _(—) _(0m), δβ_(HE) _(—) _(2m), must be maximizedto achieve large resonant wavelength separation. The above equationsindicate that this maximization is achieved by maximizing the quantitiesI₁ and I₂ and these, in turn, are maximized by a fiber design with large(scalar) LP_(1m) modal power close to the waveguide transition regions(such as steep index steps, or any other region where the gradient ofthe index profile is large).

Referring to FIG. 4 with these characteristics in mind, it is shown thatthe LP₁₁ mode, illustrated by plot 15, has both a large intensity, aswell as a large intensity gradient close to the steep index step of highindex ring 14. Additionally, a useful device should also exhibit lowinsertion loss, which requires that the fundamental mode (i.e., the modeused to couple into and out of the device) have a nominally Gaussianshape so as to correspond with like-profiled optical devices at theinput and output of the MIG. To satisfy this requirement, core region 10should be sufficiently large (or high enough in index) so that thefundamental LP₀₁ mode resides essentially in central core region 10.Therefore, such an inventive fiber can be defined in terms of refractiveindex differences and radial values as follows: (1) the refractive indexdifference Δn_(r) of ring 14 should be greater than 0.015, with asufficiently steep “inner” index step toward trench 12. The inner indexstep is defined by the spatial extent S over which the index changesfrom the value associated with ring 14 to the value associated withtrench 12, where S should be no greater than one micron; and (2) thecentral core region refractive index should be approximately 75% of thevalue of the ring refractive index, that is, the central core radius andrefractive index should be large enough such that the fundamental LP₀₁is essentially completely supported within central core region 10,without being so large that the antisymmetric LP_(1m) will also beguided within central core region 10.

FIGS. 5( a)-(c) illustrate experimentally measured properties of a fiberformed in accordance with the present invention, where FIG. 5( a) showsthe measured refractive index profile values for Δn_(c), Δn_(t) andΔn_(r). FIG. 5( b) illustrates the phase matching curves (grating periodas a function of wavelength) of the fiber design of FIG. 5( a) for theTE₀₁, HE₂₁ and TM₀₁ modes, where it is obvious that these three curvesare vastly separated. The large vector perturbation terms, leading tolarge differences in propagation constants and hence the phase matchingcurves of FIG. 5( b), is indeed the design objective of the presentinvention. FIG. 5( c) illustrates experimentally measured spectra forgratings with periods ranging from 742 μm to 850 μm, as induced in afiber with the profile of FIG. 5( a). The grating may be induced, forexample, by pressing the fiber between a corrugated metal block (withthe period of the corrugations defining the grating period) and a rubberpad. Referring to FIG. 5( c), three distinct resonant peaks are evidentin each spectrum, with the center peak (associated with the HE₂₁ mode)being the strongest. This peak is the strongest since, as discussedabove, the HE mode is polarization insensitive. The remaining pair ofresonances (TE₀₁ and TM₀₁) are relatively weak since no more thanone-half of the total available signal power will ever exist in aparticular polarization. This result thus confirms the polarizationsensitivity phenomena in microbend induced fiber gratings. It is to benoted that the wavelength separation between the HE mode and the TE/TMmodes is on the order of 60 nm, the relatively large value desired tomaintain the polarization insensitivity over the operating range of thedevice at any given time.

The significance of the polarization state of the input signal on thepresence of the TE and TM modes is evident from the values illustratedin FIG. 6, which contains a plot of transmitted intensity for an inputsignal controlled to pass through a MIG with a set of parallelpolarization states, denoted as “SOP ∥” and “SOP ⊥”. As shown, where theinput state of polarization corresponds to the TE₀₁ mode (SOP ∥), onlythe TE₀₁ and HE modes are excited, whereas for light of orthogonalpolarization (SOP ⊥) only the HE and TM₀₁ resonant peaks will be found.In accordance with the teachings of the present invention, the HE mode,being polarization insensitive, will have a resonant peak under eitherextreme polarization state. Additionally, FIG. 6 illustrates that theoptimized fiber design of the present invention yields significantlylarge wavelength splitting, on the order of 76 nm between the TE₀₁ andHE modes, and on the order of 67 nm between the TM₀₁ and HE modes.

FIGS. 7( a) and (b) contain the relative index plots and phase matchingcurves for another fiber formed in accordance with the presentinvention, where the fiber design associated with FIGS. 7( a) and (b)also exhibits a “minima” along each phase curve. As discussed in myabove-referenced co-pending application, this minima is defined as a“turn-around point” (TAP). Referring to FIG. 7( b), the grating periodfor an exemplary polarization insensitive MIG supporting only theodd/even HE modes is illustrated as horizontal line 72, where thisparticular arrangement has a grating period Λ of approximately 485 μm.It has previously been demonstrated that when the fiber grating periodis chosen to couple the phase matching curve at the TAP (the “TAPresonance condition”), large bandwidth mode coupling is achieved. Thepresence of a TAP is thus indicative of a relatively large bandwidth andspectrally flat resonance over bandwidths as large as 100 nm for theassociated microbend-induced fiber grating, characteristics that areoften crucial in the design of the fiber grating.

As mentioned above, since the polarization insensitivity of the MIG ofthe present invention is associated with a proper design of the fiber interms of refractive index differences and radial dimensions, anyappropriate method for inducing microbends can be used to form a gratingof the desired period. FIGS. 8( a)-(d) illustrate a set of fourexemplary methods of introducing the microbends in the fiber, where FIG.8( a) illustrates the use of a corrugated plate 30 and a rubber pad 32(described above) and the grooves within corrugated plate 30 exhibit theperiodicity desired to be induced into the fiber. FIG. 8( b) is avariation of this arrangement, in this case using a pair of corrugatedblocks 34 and 36 that are aligned such that their teeth fit into eachother's grooves, as shown. In this case, as the fiber is pressed betweenthe plates, the microbend grating is induced. Therefore, a lowerpressure may be used to form the same pattern as that in the fiber ofFIG. 8( a). FIG. 8( c) illustrates an acousto-optic arrangement, where apiezoelectric transducer 38 (connected to an RF power source, not shown)is used to propagate an acoustic wave along the fiber, which in turnresults in periodic microbends with a period inversely proportional tothe frequency of the RF power source and amplitude directly proportionalto the power of the RF source. Thus, both the grating period andamplitude may be adjusted with this acousto-optic arrangement. FIG. 8(d) illustrates an exemplary microbend-induced fiber grating where theperturbations in the fiber have been permanently formed, using any ofthe well-known techniques of the art, including periodic arcing with asplicer, or periodic ablation with a CO₂ laser. The embodiments of FIGS.8( a)-(d) are meant to be exemplary only, and various other techniquesof introducing microbends into a fiber section formed in accordance withthe present invention may be used to form a polarization-insensitivemicrobend-induced fiber grating.

There exists a variety of different optical systems and subsystems thatmay utilize a polarization-insensitive MIG as formed in accordance withthe present invention. FIG. 9 illustrates a cascaded arrangement of aplurality of MIGs 40-1 through 40-5, where each separate grating may beeither a dynamic (adjustable) or static device. In particular, acascaded set of such gratings may be used to create a spectrally flatoutput, particularly when the input is from an erbium-doped fiberamplifier whose gain spectrum is highly wavelength dependent. Usingdynamic filters further allows the characteristics of each grating to betuned in real time to respond to changes in the input signal spectrum.Graph A in FIG. 9 depicts the input signal spectrum, and graph B depictsthe output signal spectrum. It is shown that by using a plurality ofcascaded MIGs of the present invention, a variety of unwanted featurespresent in an input signal spectrum may be virtually eliminated and thespectrum significantly flattened over a large spectral range. Loops 42between adjacent MIGs 40 signify mode-stripping action to radiate theLP₁₁ mode out of the signal path through each individual MIG 40. Thismode stripping can be achieved, for example, by bending the fiber, usingin-fiber tapers or splices to single mode fibers.

FIG. 10( a) illustrates an exemplary variable optical attenuator (VOA)50 that may be formed in accordance with the present invention byconfiguring a microbend-induced fiber grating with a TAP condition, asdiscussed above, to provide an (adjustable) large, spectrally flatattenuation in the grating 52. The positioning of a VOA 50 at the inputof an optical receiver 53 allows for cost effective optimizing of thebit error rate at the receiver by controlling the received power. Whensuch a device is used to follow a continuous wave (CW) lightsource 55,as illustrated in FIG. 10( b), it may function as a low cost, low speeddata modulator, or used to impart low speed monitoring tones on top ofan input signal. A variable optical attenuator formed from a MIG of thepresent invention may also be used with an optical amplifier 54, asshown in FIG. 10( c), where the attenuator helps to maintain amplifier54 at a constant saturation level, thus reducing any transient changesin the noise figure or gain spectrum.

The polarization-insensitive microbend-induced fiber grating of thepresent invention can also be used as a switch, since it is capable ofmoving light between two spatial modes in a polarization insensitivefashion. FIG. 11( a) illustrates this application of the presentinvention, illustrating the operation of one such exemplary switch 60 inthe “cross” or “bar” state. Additionally, well-known techniques tocouple the LP₁₁ mode in a two-mode fiber to the fundamental mode of aseparate single mode fiber, through (for example) a fused fiber coupler,may be used in conjunction with a MIG 62 of the present invention toform a four-port 2×2 optical switch in which all of the inputs andoutputs are associated with the fundamental mode of the fiber. FIG. 11(b) illustrates this concept, with a pair of fused fiber couplers 64 and66 disposed on either side of MIG 62.

The microbend-induced fiber gratings of the present invention may beused as either static or dynamic mode converters, which are key elementsin building dispersion compensators and delay lines that operate in ahigher order mode of a fiber. FIG. 12 illustrates this concept, whereFIG. 12( a) illustrates a static dispersion compensator 67 thatcomprises a first MIG 68-1 at the input of a higher order mode (HOM)fiber 70 and a second MIG 68-2 at the output of HOM fiber 70. A tunabledispersion compensator 80 is illustrated in FIG. 12( b), where aplurality of tunable MIGs 82-1, 82-2, . . . are disposed in series, withdiffering lengths of HOM fiber 84 disposed therebetween.

Various other devices, subsystems and systems, as well as various otherparticular fiber designs, are considered to fall within the spirit andscope of the present invention as directed to a polarization-insensitivemicrobend-induced fiber grating. All deviations from the specificteachings as contained within the specification that rely on theinventive principles and their equivalents through which the art hasbeen advanced are properly considered to fall within the scope of thepresent invention as defined by the claims appended hereto.

1. A method of making a polarization insensitive optical fiber for usein mode conversion, the method comprising the steps of: forming acentral core region of a material having a predetermined refractiveindex, defined as a refractive index difference Δn_(c) as measured withrespect to the refractive index of silica, the central core regionformed to a predetermined thickness; forming a trench area to surroundthe central core region, the trench area formed from a material having arefractive index difference Δn_(t) less than the refractive indexdifference of the central core region; and forming a ring area tosurround the trench area, the ring area formed from a material having arefractive index difference (Δn_(r)) greater than 0.015, with the formedthicknesses of the trench area and ring area controlled such that atransition of no greater than one micron is formed between therefractive index difference of the trench area and the ring area, thecore region refractive index difference (Δn_(c)) being maintained to beapproximately three-quarters the value of the refractive index of thering area (i.e., Δn_(c)≈0.75*Δn_(r)), the sub-micron transition andradial gradient in refractive index resulting in splitting the resonantwavelengths of the polarization-dependent TE and TM modes from theresonant wavelength of the polarization insensitive odd/even HE mode. 2.A method of making a mode converter for use in a higher order mode of anoptical fiber, the method comprising the steps of: forming an inputmicrobend induced fiber grating and an output microbend induced fibergrating by performing the following steps for each grating: forming acentral core region of a material having a predetermined refractiveindex, defined as a refractive index difference Δn_(c) as measured withrespect to the refractive index of silica, the central core regionformed to a predetermined thickness; forming a trench area to surroundthe central core region, the trench area formed from a material having arefractive index difference Δn_(t) less than the refractive indexdifference of the central core region; and forming a ring area tosurround the trench area, the ring area formed from a material having arefractive index difference (Δn_(r)) greater than 0.015, with the formedthicknesses of the trench area and ring area controlled such that atransition of no greater than one micron is formed between therefractive index difference of the trench area and the ring area, thecore region refractive index difference (Δn_(c)) being maintained to beapproximately three-quarters the value of the refractive index of thering area (i.e., Δn_(c)≈0.75*Δn_(r)), the sub-micron transition andradial gradient in refractive index resulting in splitting the resonantwavelengths of the polarization-dependent TE and TM modes from theresonant wavelength of the polarization insensitive odd/even HE mode;and coupling the input microbend induced fiber grating to a first end ofa higher-order mode optical fiber; and coupling the output microbendinduced fiber grating to a second, opposing end of the higher-order modeoptical fiber, forming a static dispersion compensator.
 3. The method asdefined in claim 2, wherein a tunable dispersion compensator is made bycascading a plurality of microbend induced fiber gratings and aplurality of sections of higher-order mode optical fiber in series,wherein the sections of higher-order mode fiber are of different lengthsand the plurality of gratings yield signal propagation in either apolarization insensitive dispersion mode or the fundamental mode torealize a tunable dispersion compensator operating in the preferredpolarization insensitive mode.